Method and apparatus for robust mode selection with low power consumption

ABSTRACT

A low power method and apparatus for selecting operational modes of a circuit. One circuit according to the teachings of the disclosed method and apparatus includes a first current limiting circuit coupled between a selector terminal and a first voltage bus. The first current limiting circuit is adapted to vary a current limit out of the selector terminal in response to a voltage on the selector terminal. The circuit also includes a second current limiting circuit coupled between the selector terminal and a second voltage bus. The second current limiting circuit adapted to vary a current limit into the selector terminal in response to the voltage on the selector terminal.

REFERENCE TO PRIOR APPLICATION

This application is a continuation of U.S. application Ser. No.10/798,751, filed Mar. 10, 2004 now U.S. Pat. No. 7,212,058.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to integrated circuits, and morespecifically, the present invention relates to integrated circuits thatare controllers for switching power supplies.

2. Background Information

It is well known that the use of integrated circuits to replace discretecomponents in electronic circuits can reduce the cost and size ofelectronic systems. The use of integrated circuits also reduces thepower required to operate those systems. A category of electronicsystems that benefits from the use of integrated circuits is electronicpower supplies. The parameters of cost, size, and power consumption areespecially important in application of these systems. Switching powersupplies are of particular interest to designers of electronic systemsbecause the highest performance is generally achieved with switchingpower supply technology.

To realize the goals of systems designers, integrated circuits forswitching power supplies should have options that allow them to be usedin a variety of different applications. The integrated circuits shouldrequire as few discrete components as possible, and they should consumethe lowest possible power in operation. An example of a desirable optionis the ability to select different switching frequencies so that anoptimal design can be achieved for different applications of a singleintegrated circuit. The package of the integrated circuit should alsohave as few terminals as possible to minimize cost.

SUMMARY OF THE INVENTION

Disclosed are methods and apparatuses that allow a selector terminal ofa circuit to select optional modes of operation while maintaining lowpower consumption with reduced sensitivity to noise. In one embodiment,a circuit according to the teachings of the present invention includes afirst current limiting circuit coupled between the selector terminal anda first voltage bus. The first current limiting circuit is adapted tovary a current limit out of the selector terminal in response to avoltage on the selector terminal. The circuit also includes a secondcurrent limiting circuit coupled between the selector terminal and asecond voltage bus. The second current limiting circuit adapted to varya current limit into the selector terminal in response to the voltage onthe selector terminal. Additional features and benefits of the presentinvention will become apparent from the detailed description, figures,and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 shows an example of a switching power supply that includes aswitching converter and an integrated circuit controller.

FIG. 2 is a diagram illustrating the principles of a technique that usesthe voltage on a terminal of an integrated circuit to select amongoptional modes of operation.

FIG. 3 shows the elements added to the technique illustrated in FIG. 2to select optional modes of operation.

FIG. 4 is a schematic illustration of functional blocks in FIG. 3.

FIG. 5 is a schematic of one embodiment of a method and apparatus toselect optional modes of operation in accordance with the teachings ofthe present invention.

FIG. 6 is a schematic of another embodiment of the method and apparatusto achieve the selection of optional modes of operation in accordancewith the teachings of the present invention.

DETAILED DESCRIPTION

An embodiment of a method to select multiple optional modes of operationof an integrated circuit is disclosed. The method uses a voltage betweena mode selector terminal and a ground reference terminal to selectoperational modes. It allows one mode of operation corresponding to noconnection at the terminal. The method allows the operational modecorresponding to no connection at the terminal to be robust in thepresence of noise while reducing the power to maintain the desiredselection in all modes of operation.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. Well-known methods related to the implementation have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

As will be disclosed in this disclosure, an operational mode of acircuit according to the teachings of the present invention is selectedwhen the selector terminal has no external connection and also when itis connected to a source of regulated voltage that can be anotherterminal on the integrated circuit. When the selector terminal is notconnected to a source of regulated voltage, the voltage on the selectorterminal is maintained within boundaries in the presence of noise byswitched current sources that absorb noise currents at the selectorterminal. The current sources change in magnitude according to thevoltage on the selector terminal. The magnitudes of the current sourcesare greatly reduced when the selector terminal is connected to anexternal voltage that is outside the voltage boundaries for theunconnected selector terminal to reduce the power consumed by theintegrated circuit.

FIG. 1 is a diagram of an example switching power supply with anintegrated circuit controller that could employ an embodiment of thepresent invention. An unregulated DC input voltage V_(IN) 100 isconverted to a regulated DC output voltage V_(out) 102 by switchingconverter 101 that is controlled by integrated circuit 117. All voltagesare with respect to the ground reference 111. The state of a single poledouble throw power switch S_(P) 103 is controlled by the signalPWM_(OUT) 107 from the integrated circuit 117. Switch S_(P) 103 couplesthe inductor 104 to the input voltage V_(IN) 100 when PWM_(OUT) on line107 is high. Switch S_(P) 103 couples one end of the inductor 104 to theground reference 111 when the signal PWM_(OUT) on line 107 is low.Capacitor 105 filters the AC current in inductor 104 to provide asubstantially DC voltage to the load 106. The frequency of the sawtoothoscillator 114 within integrated circuit controller 117 determines therate of switching.

A plurality of functional terminals 118 on the integrated circuit 117can be coupled to operate the various functions of integrated circuit117 in a desired manner. For example, functional terminals 118 might setthe frequency of the oscillator 114. The frequency could take on anynumber of values depending on the number of terminals used to implementthe selection and the stimulus required at functional terminals 118. Astimulus could take the form of a voltage, a current, or the equivalentthat could be determined by a discrete component connected between twoterminals. A single terminal could select multiple options by connectionto multiple levels of voltage or current. In any case, it is desired inone embodiment that the method of selection does not require the use ofdiscrete components, and that the selected options maintain the lowestpossible power consumption of the integrated circuit.

Integrated circuit 117 senses the output voltage V_(OUT) 102 of theswitching converter 101 at a terminal V_(SENSE) 109 with respect to aground terminal GND 108. In one embodiment, an error amplifier 110within the integrated circuit 117 amplifies the difference between thevoltage at terminal V_(SENSE) 109 and a reference voltage 116 internalto the integrated circuit 117. A comparator 112 compares the errorvoltage output 113 of error amplifier 110 to the sawtooth voltage V_(F)115 that is an output of the oscillator 114. The output 107 of thecomparator 112 is high when the error voltage 113 is greater thansawtooth voltage 115. The output 107 of comparator 112 is low when theerror voltage 113 is less than the sawtooth voltage 115. Thus, theperiodic switching of power switch S_(P) 103 is modulated by theintegrated circuit 117 in a manner to regulate the output voltageV_(OUT) 102. There are many ways to implement the function of the switchS_(P) 103 with semiconductor devices, such as for example twotransistors, or a transistor and a diode. The combination of switchingconverter 101 and integrated circuit controller 117 in FIG. 1 is justone example of many different circuits that could be used in switchingpower supplies.

FIG. 2 illustrates one principle of voltage comparison that can be usedin the design of integrated circuit controllers to select amongdifferent optional modes of operation, shown in the context of powersupply controller 117. All voltages are measured with respect to theground reference terminal 108. A mode select terminal 206 is coupled toa plurality of voltage comparators 202 that compare the voltage at 206to the threshold voltages 208. The comparators 202 are coupled to adecoder circuit 203 that produces an output 209 to indicate which modeof operation has been selected by the voltage at the mode selectterminal 206.

In the arrangement illustrated in FIG. 2, a plurality of N voltagecomparators 202 can determine N+1 distinct modes of operation. Eachcomparator determines if the voltage at the mode select terminal 206 ishigher or lower than one of the threshold voltages 208. With thisarrangement, N threshold voltages create N+1 possible selections.Decoder 203 enables the mode of operation that corresponds to thevoltage at mode select terminal 206.

The user sets the voltage on the mode select terminal 206 at the valueappropriate for the desired optional mode of operation. One appropriatevalue could be zero, corresponding to a connection of the mode selectterminal 206 to the ground reference terminal 108. Other appropriatevalues could be from internally generated voltages that are available atother functional terminals of the integrated circuit, such as voltageVDD at terminal 200 from internal voltage source 201, and VCC atterminal 204 from internal voltage source 205 in this illustration. Forpurposes of this disclosure, it is noted that voltage VDD at terminal200 may be referred to as a first voltage bus and the ground referenceterminal 108 may be referred to as a second voltage bus. An appropriatevoltage could also be obtained from an external circuit as representedby voltage VX at node 207 in FIG. 2.

All the voltages may be obtained by direct connection of the mode selectterminal 206 to a node in the power supply or to a terminal of thecircuit. No discrete components are required to select an optional modeof operation. To obtain the maximum number of optional modes that may beselected with the mode select terminal 206, the unconnected or opencircuit state of terminal 206 should also correspond to a predeterminedmode of operation. Therefore, when mode select terminal 206 has noexternal connection, its voltage must be bounded between thresholds ofthe comparators 202.

It is appreciated that the arrangement in FIG. 2 cannot meet therequirement for no external connection on the mode select terminal 206because the voltage at 206 is undetermined when 206 has no externalconnection. Moreover, the unconnected mode select terminal 206 is a highimpedance node, whose voltage is strongly influenced by noise currents.Switching power supplies have electric and magnetic fields that caninject noise currents at the mode select terminal 206. Small noisecurrents with the high impedance at the mode select terminal 206 willproduce voltages that cause the unintended selection of modes.

FIG. 3 shows additional internal circuits 300 and 301 that may be usedto determine the voltage at 206 when select terminal 206 has no externalconnection. The internal circuits 300 and 301 also allow the selectionof optional modes with mode selection terminal 206 while limiting powerconsumption of the integrated circuit 117. Furthermore, the circuits 300and 301 must maintain the voltage on the mode select terminal 206between boundaries in the presence of noise when there is no externalconnection to the mode select terminal 206.

One technique to sense multiple states through a terminal of anintegrated circuit, including a state corresponding to no connection atthe terminal, requires the circuit to sense the current at the terminal,and to determine if the current is sourced to the terminal or sinkedfrom the terminal. This technique, however, fails to reduce the powerconsumption of circuitry that is coupled to the terminal.

FIG. 4 shows an embodiment of circuits 300 and 301 including additionalcircuitry that senses the voltage directly on the mode select terminal206. The embodiment illustrated in FIG. 4 does not require sensing ofcurrent into or out of the mode select terminal 206. Accordingly, theembodiment illustrated in FIG. 4 provides robust limits on the voltageat the mode select terminal 206 when terminal 206 has no externalconnection, but lacks the desired property of reduced power consumption.

In the embodiment illustrated in FIG. 4, circuits 300 and 301 arecurrent limiting circuits that are engaged by transistor switches 401and 402 respectively. For purposes of this disclosure, a switch may bedefined as a device that controls the passage of electric current. Aswitch can allow any magnitude of current from zero current to themaximum current available to it, depending on its stimulus. As shown inFIG. 4, transistors 401 and 402 have their respective gates held atrespective voltages V_(G1) 414 and V_(G2) 413 by respective internalvoltage sources 408 and 409. In one embodiment, V_(G1) is less than orequal to V_(G2). Transistors 401 and 402 do not conduct as long as thevoltage on terminal 206 is between a lower voltage boundary and an uppervoltage boundary. In one embodiment, the lower voltage boundary isV_(G1) minus the gate threshold voltage of transistor 401 while theupper voltage boundary is V_(G2) plus the gate threshold voltage oftransistor 402. Noise currents that move the voltage on 206 to eitherthe upper voltage boundary or the lower voltage boundary will causeeither 401 or 402 to conduct. The current in transistor 401 is limitedto I₁ by a current limit circuit that is current source 400. The currentin transistor 402 is limited to I₂ by a current limiting circuit that iscurrent source 403. The voltage on terminal 206 will not go higher thanthe upper voltage boundary or lower than the lower voltage boundary aslong as the noise current does not exceed respective current limits I₂of current source 403 or I₁ of current source 400. The open circuitcondition is robust because the voltage on the mode select terminal 206has to go substantially higher than the upper voltage boundary orsubstantially lower than the lower voltage boundary to select adifferent mode of operation.

In one embodiment, transistors 404 and 407 conduct when the voltage onthe mode select terminal 206 is between the lower voltage boundary andthe upper voltage boundary. Thus, the operational mode corresponding toan open circuit on terminal 206 is set by the conduction of bothtransistors 404 and 407. Thus, transistors 404 and 407 behave likevoltage comparators. The reference voltage for the comparator comprisedof transistor 404 is VDD minus the gate threshold voltage of transistor404. The reference voltage for the comparator comprised of transistor407 is the ground reference 108 plus the gate threshold of transistor407. To select a different mode of operation, the voltage on the modeselect terminal 206 must go higher than VDD minus the gate thresholdvoltage of transistor 404 or lower than the ground reference plus thegate threshold voltage of transistor 407. Noise current can cause thevoltage on the mode select terminal 206 to go high enough or low enoughto change the mode of operation only if the noise current enteringterminal 206 is greater than current limit I₂ of current source 403 orif the noise current leaving terminal 206 is greater than current limitI₁ of current source 400. High immunity to noise is achieved by makingI₁ and I₂ large.

In the circuit of FIG. 4, current source 400 has the single constantvalue I₁ and current source 403 has the single constant value I₂. Adisadvantage with making I₁ and I₂ large is that the mode selectterminal 206 will conduct the large current I₁ or I₂ when the othermodes of operation are selected. Large current requires proportionallylarge power from the internal or external voltage source that providesthe current, and the circuit that receives the current consumes thepower. It is desirable to consume as little power as possible and at thesame time provide high immunity to noise.

FIG. 5 shows one embodiment of a circuit that solves the problem oflarge power consumption while maintaining high immunity to noise on themode select terminal 206. In one embodiment, the circuit of FIG. 5 maybe a selector circuit, which may be included in an integrated circuitdevice. In one embodiment, the integrated circuit device may be acontroller of a switching power supply, such as for example one similarto that illustrated in FIG. 1. As illustrated in the circuit embodimentin FIG. 5, circuit 300 is coupled between selector terminal 206 and afirst voltage bus, shown in the depicted embodiment as VDD 200.Similarly, circuit 301 is coupled between selector terminal 206 andground reference terminal 108. It is appreciated that VDD 200 and groundterminal 108 may be considered as first and second voltage busses. Aswill be discussed, in operation, circuit 300 is adapted to vary acurrent limit out of selector terminal 206 in response to the voltage atselector terminal 206 and circuit 301 is adapted to vary a current limitinto selector terminal 206 in response to the voltage at selectorterminal 206. In one embodiment, the circuit also includes a pluralityof voltage comparators 202 coupled to selector terminal 206 and coupledto circuits 300 and 301. In one embodiment, the plurality of voltagecomparators are also coupled to a decoder circuit such as for exampledecoder circuit 203.

The embodiment in FIG. 5 also shows that circuit 300 includes a variablecurrent source 400 and that circuit 301 includes a variable currentsource 403. It is noted that whereas the current limiting circuit 400 inFIG. 4 is fixed at a single value I₁, the current limiting circuit 400in FIG. 5 is a variable current limit circuit with at least two possiblevalues: a high value and a low value. Similarly, whereas the currentlimiting circuit 403 in FIG. 4 is fixed at a single value I₂, thecurrent limiting circuit 403 in FIG. 5 is also a variable current limitcircuit with at least two possible values: a high value and a low value.In each current limiting circuit 400 and 403 in FIG. 5, the high valueis engaged to obtain high immunity to noise when the voltage on the modeselect terminal 206 has no external connection. In one embodiment, thelow value is engaged when the mode select terminal 206 is connected to ahard voltage source that does not require the high current for immunityto noise. The low value needs to be only large enough to return thevoltage on the mode select terminal 206 to between the two boundariesthat are defined by V_(G1), V_(G2), and the respective gate thresholdvoltages of transistors 401 and 402 when the mode select terminal isdisconnected from the hard voltage source.

In the embodiment of FIG. 5, the variable current limiting circuit 400includes two fixed current sources. One fixed current source is currentsource 501, which has a magnitude I_(L1). The other fixed current sourceis current source 500, which has a magnitude I_(H1). The magnitude ofcurrent source 501 is substantially less than the magnitude of currentsource 500. Transistor switch 502 determines whether or not the highmagnitude current source 500 adds to the low magnitude current source501.

Similarly, the variable current limiting circuit 403 in FIG. 5 includestwo fixed current sources. One fixed current source is current source503, which has a magnitude I_(L2). The other fixed current source iscurrent source 504, which has a magnitude I_(H2). The magnitude ofcurrent source 503 is substantially less than the magnitude of currentsource 504. Transistor switch 505 determines whether or not the highmagnitude current source 504 adds to the low magnitude current source503.

In one embodiment, the variable current limiting circuit 400 in FIG. 5switches from a high current to a low current when the voltage on themode select terminal 206 is less than a gate threshold voltage oftransistor 407 above the ground reference. This voltage near the groundreference on the mode select terminal 206 selects a different mode ofoperation. The switching between high and low values is accomplished bytransistor 502 that is coupled to the output 412 of a voltage comparatorcomprised of transistor 407. The high current limit that gives immunityto noise is not required under this condition because in practice thevoltage on the mode select terminal 206 will be fixed by an externalconnection to a hard voltage such as for example the ground referenceterminal 108.

Similarly, the variable current limiting circuit 403 in the embodimentof FIG. 5 switches from a high current to a low current when the voltageon the mode select terminal 206 is greater than the voltage VDD atterminal 200 minus the gate threshold voltage of transistor 404. Thisvoltage near VDD on the mode select terminal 206 selects a differentmode of operation. The switching of the current source between high andlow values is accomplished by transistor 505 that is coupled to theoutput 411 of a voltage comparator comprised of transistor 404. The highcurrent that gives noise immunity is not required under this conditionbecause in practice the voltage on mode select terminal 206 will befixed by an external connection to a regulated voltage such as forexample VDD terminal 200.

FIG. 6 is a schematic illustrating another embodiment of the presentinvention in which inverters 601, 602, 603 and 604 are added to theembodiment of FIG. 5. In the illustrated embodiment, inverters 601, 602,603 and 604 help assure that the signals at the lines 506, 606, 605 and507 respectively have sufficient drive for reliable operation. In theembodiment of FIG. 6, the gate 414 of transistor 401 and the gate 413 oftransistor 402 are coupled to a reference voltage 600, which in oneembodiment is half the power supply voltage VDD 200. The value of halfthe power supply voltage VDD for the reference voltage 600 in theembodiment of FIG. 6 gives the greatest immunity to noise when the modeselect terminal 206 has no external connection.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A method, comprising: receiving a voltage at a selector terminal ofan integrated circuit; selecting between multiple modes of operation ofthe integrated circuit, wherein a first mode of operation is selectedwhen there is no connection to the selector terminal external to theintegrated circuit and wherein a second mode of operation is selectedwhen the voltage at the selector terminal is fixed by a regulatedvoltage external to the integrated circuit; selecting a fixed currentlimit value from a plurality of fixed current limit values in responseto the received voltage on the selector terminal; limiting a currentthrough the selector terminal to the selected fixed current limit value;and switching a power switch with the integrated circuit to regulate anoutput of a switching power supply.
 2. The method of claim 1, whereinthe plurality of fixed current limit values comprises a high fixedcurrent limit value and a low fixed current limit value.
 3. The methodof claim 2, wherein the high fixed current limit value is greater thanthe low fixed current value.
 4. The method of claim 2, wherein limitingthe current through the selector terminal comprises limiting a currentout of the selector terminal.
 5. The method of claim 2, wherein limitingthe current through the selector terminal comprises limiting the currentout of the selector terminal to the high fixed current limit value whenthe first mode of operation of the integrated circuit is selected. 6.The method of claim 5, further comprising switching from the high fixedcurrent limit value to the low fixed current limit value when thevoltage received at the selector terminal is less than a thresholdvoltage.
 7. The method of claim 2, wherein limiting the current throughthe selector terminal comprises limiting the current out of the selectorterminal to the low fixed current limit value when the second mode ofoperation of the integrated circuit is selected.
 8. The method of claim2, wherein limiting the current through the selector terminal compriseslimiting a current into the selector terminal.
 9. The method of claim 2,wherein limiting the current through the selector terminal compriseslimiting the current into the selector terminal to the high fixedcurrent limit value when the first mode of operation of the integratedcircuit is selected.
 10. The method of claim 9, further comprisingswitching from the high fixed current limit value to the low fixedcurrent limit value when the voltage received at the selector terminalis greater than a threshold voltage.
 11. The method of claim 2, whereinlimiting the current through the selector terminal comprises limitingthe current into the selector terminal to the low fixed current limitvalue when the second mode of operation of the integrated circuit isselected.
 12. The method of claim 1, wherein selecting a fixed currentlimit value from a plurality of fixed current limit values comprises:selecting a first fixed current limit value from a first plurality offixed current limit values in response to the received voltage on theselector terminal; and selecting a second fixed current limit value froma second plurality of fixed current limit values in response to thereceived voltage on the selector terminal.
 13. The method of claim 12,wherein limiting a current through the selector terminal to the selectedfixed current limit value comprises: limiting a current out of theselector terminal to the selected first fixed current limit value; andlimiting a current into the selector terminal to the selected secondfixed current limit value.
 14. The method of claim 1, further comprisingbinding the voltage at the selector terminal between an upper voltageboundary and a lower voltage boundary when there is no externalconnection to the selector terminal.
 15. The method of claim 1, whereinthe integrated circuit device is a controller in the switching powersupply.
 16. The method of claim 11, further comprising switching a powerswitch coupled to the integrated circuit to regulate an output of theswitching power supply responsive to the voltage received at theselector terminal.
 17. The method of claim 1, further comprisingproducing an output with a decoder circuit included in the integratedcircuit to indicate which mode of operation has been selected by thevoltage at the mode select terminal.
 18. The method of claim 1, whereinlimiting a current through the selector terminal to the selected fixedcurrent limit value comprises limiting a noise current through theselector terminal when there is no connection to the selector terminalexternal to the integrated circuit.